The present invention relates to films that are receptive to solvent-based inkjet inks and methods of printing onto such films. More specifically, the present invention relates to melt-processed films that are receptive to solvent-based inkjet inks and methods of printing onto such films. A variety of polymeric sheets may be prepared including various sheeting for signage and graphic films for advertising and promotional displays.
A variety of print methods have been employed for imaging various sheet materials. Commonly employed print methods include gravure, off-set, flexographic, lithographic, electrographic, electrophotographic (including laser printing and xerography), ion deposition (also referred to as electron beam imaging (EBI)), magnetographics, inkjet printing, screen printing, and thermal mass transfer. More detailed information concerning such methods is available in standard printing textbooks.
One of ordinary skill in the art appreciates the differences in these various print methods and recognizes that a combination of ink and receiving substrate that results in high image quality in one printing method often exhibits an entirely different image quality with another print method. For example, in contact printing methods such as screen-printing, a blade forces the ink to advance and wet the receiving substrate. Image defects are typically due to a subsequent recession of the ink contact angle with the substrate. In the case of non-contact printing methods, such as inkjet printing, the individual ink drops are merely deposited on the surface. In order to achieve good image quality, the ink drops need to spread, join together, and form a substantially uniform, leveled film. This process requires a low advancing contact angle between the ink and the substrate. For any given ink/substrate combination, the advancing contact angle is typically significantly greater than the receding contact angle. Accordingly, ink/substrate combinations that result in good image quality when printed with contact methods such as screen printing, often exhibit insufficient wetting when imaged with non-contact printing methods such as inkjet printing. Insufficient wetting results in low radial diffusion of the individual ink drops on the surface of the substrate (also referred to as xe2x80x9cdot gainxe2x80x9d), low color density, and banding effects (e.g., gaps between rows of drops).
Another important difference between screen-printing and inkjet printing is the physical properties of the ink. Screen printing ink compositions typically contain over 40% solids and have a viscosity of at least two orders of magnitude greater than the viscosity of inkjet printing inks. It is not generally feasible to dilute a screen printing ink to make it suitable for inkjet printing. The addition of large amounts of low viscosity diluents drastically deteriorates the ink performance and properties, particularly the durability. Further, the polymers employed in screen printing inks are typically high in molecular weight and exhibit significant elasticity. In contrast, inkjet ink compositions are typically Newtonian.
Inkjet printing is emerging as the digital printing method of choice due to its good resolution, flexibility, high speed, and affordability. Inkjet printers operate by ejecting, onto a receiving substrate, controlled patterns of closely spaced ink droplets. By selectively regulating the pattern of ink droplets, inkjet printers can produce a wide variety of printed features, including text, graphics, holograms, and the like. The inks most commonly used in inkjet printers are water-based or solvent-based. Water-based inks require porous substrates or substrates with special coatings that absorb water.
In one aspect, the invention provides an image receptor medium comprising a porous, voided, melt-processed image receptive film layer comprising a) an immiscible blend of a semicrystalline polymer component and at least one ink absorptive polymer component and b) at least one inorganic filler. The image receptor medium may further comprise one or more additional layers such support or adhesive layers. The image receptor medium is a suitable substrate for non-contact ink-jet printing methods, and is also useful for contact printing methods. The present invention provides an image receptor medium having good color density, as measured by densitometry, and good resolution, particularly two color resolution, as measured by printing rows of parallel lines of alternating colors, the lines of the first color printed at 400% ink lay down (i.e. four passes by an ink-jet printer) and the lines of the second color being printed at 200% ink lay down, each line being printed at one-fiftieth of an inch thickness, and the lines are readily resolved by eye and do not bleed together. As used herein xe2x80x9cink lay downxe2x80x9d means the amount of ink delivered to the surface of a substrate per unit of line length.
The ink absorptive polymer component may be selected using an Inclined Ink Trail test. Briefly, the test value is the ratio of the distance traveled by an ink droplet on an inclined test substrate to that of a poly(vinyl chloride) substrate. In this test, a film of a candidate ink absorptive polymer is mounted on an inclined substrate, an ink droplet, typically 10 microliters, is applied to it and the distance the droplet runs down the inclined surface is measured. This measured distance is then compared to the distance measured on a control poly(vinyl chloride) substrate. Useful ink absorptive polymer components are defined by the equation:
IT#=Y/X=0.75 to 2.0, 
Where xe2x80x9cIT#xe2x80x9d is the inclined ink test value, X is the distance traveled by a droplet on an inclined poly(vinyl chloride) substrate and Y is the distance traveled on the candidate ink absorptive polymer component substrate.
It has been found if the inclined ink test value is less than 0.75, then the color density of the ultimate image receptive medium is poor, as a result of the solvent of the ink being absorbed too quickly. Conversely, if the inclined ink test value is greater than 2.0, the resolution of the ultimate image receptive medium is poor, as result of the ink absorptive polymer having no affinity for the solvent of the ink. Preferably, the inclined ink test value will be 0.9 to 1.25.
It will be understood with respect to the above description that candidate ink absorptive polymer components are tested using the inks specified for a particular printing process, and that depending on the inks employed, different ink absorptive polymer components may be selected. For example, in an ink jet printing process, ink jet printing inks would be used in evaluating candidate ink absorptive polymer components. Further, ink absorptive polymer components comprising blends of polymers may also be tested using the Inclined Ink Trail Test.
Although many poly(vinyl chloride) films may be used as a control in evaluating candidate ink absorptive polymer components, it has been found that the differences in the test results, i.e the X value, are generally minor, varying less than 10% between different commercially available films. One useful control film is 3M 180-10 Commercial Graphic vinyl film, available from the 3M Company. Another useful control film is MPI 1005 vinyl film, available from Avery-Dennison Graphics Div., Additional details on the Inclined Ink Trail Test are provided in the experimental section.
Many useful ink absorptive polymer components have a Hildebrand Solubility Parameter of said absorptive additive within about 3.1 (MPa)1/2 of the solvent of the ink, wherein the image receptive layer has an ink solvent absorption of at least 50% greater than a film of semicrystalline polymer component alone.
In another aspect, the invention further comprises an image receptor medium comprising biaxially oriented, melt-processed image receptive film comprising a) at least one immiscible blend of a semicrystalline polymer component, an ink absorptive polymer component and b) at least one inorganic filler. The ink absorptive polymer component is, for example, an ethylene vinyl acetate carbon monoxide terpolymer. The image receptor medium of such a construction may further comprise one or more additional layers such as support or adhesive layers.
In another aspect, the invention provides a method of printing with an inkjet printer comprising the step of jetting a solvent-based inkjet ink onto an image receptor medium comprising a biaxially oriented, melt-processed image receptive layer comprising a) an immiscible blend of a semicrystalline polymer component and an ink absorptive polymer component and b) an inorganic filler.
In yet another aspect, the invention provides an imaged ink receptor media comprising an image receptive layer of the invention having an image printed thereon. The articles of the invention are useful as an intermediate or as a finished product for signage and graphic films.